High Speed Intermittent Miss

[Bob Chadwick]

Pete,

Advance curve does come in a little later than some but I was concerned at the lower curve point indicated with the next lighter set of springs. I think they brought it in at 2800 and I had gotten some feedback about pre-ignition. Once I get this problem licked, I may advance it a little more aggreesively.

Distributor Vacuum Advance Control Units Specs and facts for GM Distributors by Lars Grimsrud.

Most GM V8 engines (not including "fast-burn" style heads), and specifically Chevys, will produce peak torque and power at wide open throttle with a total timing advance of 36 degrees (some will take 38). Also, a GM V8 engine, under light load and steady-state cruise, will accept a maximum timing advance of about 52 degrees. Some will take up to 54 degrees advance under these conditions. Once you advance the timing beyond this, the engine/car will start to "chug" or "jerk" at cruise due to the over-advanced timing condition. Anything less than 52 degrees produces less than optimum fuel economy at cruise speed.

Lars has a pretty good reputation in this area. I might be a little high but are you sure about 38 degrees combined?

I'll check this weekend at NAPA and see if I can get a fuel pressure gauge.

 

[lone73]

Intermittent miss

Had a miss prob exactly like yours about 6 years ago w/orig motor. I tried everything under the sun. New distributor, plugs, wires, valve springs, did a compression check, Tried an HEI distributor (coil in cap), tried 3 different rebuilt carbs, tried changing the timing curve. Nothing even made a difference.

know what finally fixed it ???
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A NEW ZZ4 !!!
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[Bob Chadwick]

OK. I'm in

 

[curtis]

Bob, Ther may not be any gas in the filter if it is a qjet, because the filter is above the float level of carb. Still sounds like fuel delivery problem to me because the problem only starts after high rpm running. If not a fliter problem , then it must me a restriction to the pickup in tank or fuel pump not up to snuff. Appears that your carburetor is running out of gas after the the high rpm stuff,

We had a race car on alcohol that did this. It would run great for a while at high rpm and load then just start to misfire. Slowed down and all OK, so picked up rpm again and OK for a while then Misfire. Our problem was too small of fuel lines to feed that much fuel to the carb. Your deal could just be a weak pump. Even new ones sometimes don't work right. I thought it might be ignition, but all that would produce the symtoms you describe are extrememly aggressive timeing (over 54 no load with vacuum at high rpm) or heat soak in the HEI module. It doesn' seem likely it is heat soak because the miss goes away before the engine cools down. Your timeing specs look perfect to me. Just like mine. replace the pump

 

[MoeJr]

Have you tried pulling the MSD and using the stock HEI? MSD's are nice but just fluff for anything but a race car. A good HEI will provide you with all the spark you need. I would try swapping the MSD out and put the HEI back in and see if the problem goes away.

 

[wishuwerehere82]

Pete,

Advance curve does come in a little later than some but I was concerned at the lower curve point indicated with the next lighter set of springs. I think they brought it in at 2800 and I had gotten some feedback about pre-ignition. Once I get this problem licked, I may advance it a little more aggreesively.


Lars has a pretty good reputation in this area. I might be a little high but are you sure about 38 degrees combined?

I'll check this weekend at NAPA and see if I can get a fuel pressure gauge.

Those are the numbers that I have always heard regarding full advance for a small block Chevy. I would never contradict Lars however positive I may seem to be on the subject.

 

[Bob Chadwick]

Have you tried pulling the MSD and using the stock HEI? MSD's are nice but just fluff for anything but a race car. A good HEI will provide you with all the spark you need. I would try swapping the MSD out and put the HEI back in and see if the problem goes away.

I'm sure it will because it wasn't there before. This is the last resort.

 

[wishuwerehere82]

Bobchad,

I worked on my buddy's 69 Z28 back in the early 70's when the Accel dual point distributor was the hot setup. It would break up at 4000 RPM and we pulled it out and bench tested it about 3 times. Still no go. We finally put the stock distributor back in and had no problems whatsoever with that one. Sometimes the hot setup isn't so hot, I guess.

 

[JohnZ]

Bobchad,54 is way too high for the advance with vacuum advance on. You may have lost an advance spring on your mechanical advance in the distributor. 54 would cause a lot of popping at high RPM's.
The max should be 38 with vacuum advance connected.

Nope - his timing map is right on the money; "total timing" (34-38 degrees at high rpm, when it won't advance any more), is measured only as initial plus centrifugal advance, with the vacuum advance disconnected. Connecting the vacuum advance adds up to 15*-16* to initial plus centrifugal when the port is exposed to manifold vacuum, and that's perfectly normal at cruise. 52*-54* advance is to be expected at cruise if the system is operating normally; as soon as you accelerate, the vacuum advance disappears immediately, and you're back to initial plus whatever the centrifugal is providing at that rpm.

 

[GerryLP]

Bobchad,

Talk about Deja Vu! I have been reading, and it all sounds very familiar; however, I agree that the symptoms could be caused by many different situations, so I won't tell you that what caused my timing issue is causing yours (unless you have welded on the vehicle since the box was installed).

HEI's are notorious for missing at high RPM, and one of the improvements developed to correct this problem is to change the rotor screws to nylon-type screws. The suspect cause is spark grounding at the rotor metal screws, and it has been documented by Davis Unified Ignitions (DUI), so all of their ignition distributors include the nylon screews. They have their claims, but the only one I can make is that the idle and high rpm miss was greatly reduced when I switched to the nylon screws. It wasn't until later, that I decided to add the MSD 6AL box. So it might be worth trying. At the time, I went to Home Depot, and I found a couple of nylon-type screws (they were about $1.50 each and DUI sells them for $0.70 cents each approx.).

IMHO, the fuel pump factor should also manifest itself (as backfiring) during hard acceleration to be a factor in cruising.

On my return leg to NM, I finally disconnected the box in Oklahoma City and reinstalled th stock module; however, along with the lack of problems (i.e. popping/backfiring/afterfiring), I also got back the mushy gas pedal and the lower gas milage, so going back to the stock module is not a permanent solution unless one is never going to hit the pedal hard at a stop light. My Accel Performance module reduced the gas pedal mushiness at high rpm somewhat, but the engine does not perform quite like having the MSD box doing all the switching. Dialing back the timing is a solution if going from a weak energy spark to a strong energy spark (see the MSD troubleshooting guides).

Along the way in troubleshooting this problem, I netted improvements by installing 300 ohm / foot ignition wires and the Brute Accel coil. When the culprit was found, the related improvements contributed to a more reliable ignition system.

GerryLP:cool

 

[Bob Chadwick]

I have been in touch with Lars and, as JohnZ has pointed out, he feels the advance is right on the money.

Lars thinks, and some others have agreed, that I've got a problem with the carb being too lean for the Edelbrock Performer intake manifold. Shortly after adding it the car was a little rough at high speed. Then when I added the more agressive advance curve, the problem was exacerbated. He recomends increasing the jets by two sizes.

I want to check with MSD tonight on one other possibliity. They usually have good customer support but they have not responded to an email and their forums have been closed. I have read that there can be problems at high speed if the polarity is reversed on the wires that connect in the distributor. The connectors are two different sizes so they can only go one way. I used their harness so this shouldn't normally be an issue but I had to extend it and, while I was careful, it is possible I put the clips on the wrong wires. There is evidently a test with the timing light and by reversing the wires, the timing will change with either the higher or the lower timig being correct. Of course I don't recall which is which and can't find the tech article.

Gerry, I'll look into the nylon screws and for the tech article at MSD on reducing the advance. I'm glad you got yours working. As I recall, you had a damaged box that MSD repaired.

All of this and I appear to have blown head gaskets on my daily driver but that is a whole other story that I don't want to get into.

 

[GerryLP]

Bob,

lol...you're right...that is the PITA of the ignition issues...fuel and electric problems can seemingly create the same situation.

MSD did not repair the box, for it was too expensive. In turn, they did offer me a cosmetic return (previously purchased unit that was consequently tested and could not be sold as new) for $65. They really took care of my problem nicely. On the downside, as you say, its hard to get them to reply by email. If the re-jetting does not fx the problem and the nylon screws do neither, then I suggest you send them the box. They seem more responsive when you do that.

GerryLP:cool

 

[tlong]

I'm on the sidelines cheering you on Bobchad. I logged some quality time in pursuit the old girl. Do you think the lightning under the hood with the lights off would tell anything? It sounds like it may be a distributor problem. Also replace regulator and battery if 17 volts as stated. Never seen the fuel filter? Dayum.

 

[Bob Chadwick]

I'm on the sidelines cheering you on Bobchad. I logged some quality time in pursuit the old girl. Do you think the lightning under the hood with the lights off would tell anything? It sounds like it may be a distributor problem. Also replace regulator and battery if 17 volts as stated. Never seen the fuel filter? Dayum.

Tim,

No arcing from the wires and the alternator was replaced. I would have guessed the distributor was well but those brighter than me are suggesting fuel.

Just a couple of comments on the discussion here:

First, at WOT and high rpm, vacuum leaks are virtually irrelevant. You can have a massive vacuum leak, and it will not cause a miss at elevated rpm and wide open throttle. Vacuum leaks tend to affect idle and part throttle operation, but get "drowned out" at WOT.

You timing numbers look good. 36 total mechanical and 54 with total combined centrifugal plus vacuum is right on the money. No issue there.

I would check a couple of things. First, set your plug gaps to .035". This will lower the resistance across the gaps and will reduce the possibility of high rpm misfire.

Water in the fuel is a common issue that will cause bizzarre problems. Run the tank close to empty and then drain it. Refill with new gas.

I didn't see what type of carb you're running, but you were talking about servicing the fuel filter - sounds like an in-line filter. Make sure you're not running 2 filters: All Q-Jets and some Holleys have in-carb filters. If you run an in-line filter in addition to the in-carb filter, you can easily run into high-rpm fuel starvation. This will appear as a "miss". If you are running an in-line filter without an in-carb filter, make sure the filter is not installed between the tank and the fuel pump: the filter needs to be installed between the pump and the carb.

A high-rpm lean-out condition from a lean jetted carb will also cause what you are seeing. Not knowing your carb type, make sure you are jetted no leaner than the stock jetting for that carb - you can even bump jetting up 2 sizes to be safe and then check your plugs for color.

If the problem persists, just yank the distributor out and install a stock HEI without the MSD system hooked up. If the problem goes away, you just found your culprit.

Bob

 

[Bob Chadwick]

Posted on the MSD Board. Let's see what they say. I purposely did not post the information about a possible carb jetting issue.

 

[MoeJr]

Bob do you have the stock QJet on that car? If so, there is no need to change jets to richen up the carb. Just change the secondary rods to get more fuel at high speed. If it is the stock 77 carb it should have rods stamped CH or CP. Buy the rods from edelbrock that are stamped CE. also may wanna change the hanger to one stamped with any of the following letters G thru L.

 

[Bob Chadwick]

Now your scaring me Moe. It is a stock Q'Jet but I have never touched it and have no idea what you are talking about. :confused How difficult is it to do.

 

[MoeJr]

Changing the secondary rods on a Qjet takes all of 2 minutes and is super easy. If you push down on your secondary air flappers of the carb you'll notice the hanger rise up as you push down. Just unscrew the holding screw on the hanger and pull the hanger and rods up. swap the rods off and reverse to put back in. It is so simple it is scary. I didn't see where ya are if you are close I would give ya a hand.

 

[Bob Chadwick]

Thanks Moe.

I'm in Atlanta. If we were close, my wife would divorce me and move back to to the warmer climates.

I pulled the wires that go into the distributor and am trying to find a path from the front of the car to the distributor away from any power carrying wires per MSD.

Make sure that the mag pickup wires are not routed with any other voltage carrying wires. They need to be isolated. If not, they can pick up some stray signals and cause a miss in the ignition system.

Do you know how hard that is on a C3? First off are there any other kinds of wires? And then all the wires seem to go through two little areas, one on each side of the radiator.

 

[curtis]

maybe you could foil wrap your wires to isolate them. sort of like the television cable has a braiding to knock out interference. something along those lines. Looks like it's time to get creative.